1. Field of the Invention
The present invention relates to circuit substrates used for various electric and electronic apparatuses and also semiconductor modules using the same. Particularly, the present invention relates to thermal conductive substrates suitable for apparatuses that are used in a field of power electronics or the like and require improved thermal radiation property.
2. Description of Related Art
Recently, as high performance and miniaturization of electronic apparatuses have been required, high density and high performance semiconductors have been sought. Consequently, circuit substrates for mounting thereof also have been required to be small and of high density. As a result, it is important to design circuit substrates taking the thermal radiation property into consideration. While many conventional printed circuit boards are made of a glass-epoxy resin, a well known technique for improving the thermal radiation property of circuit substrates includes using a metal base substrate of a metal such as copper or aluminum and forms a circuit pattern on one face or both faces of this metal substrate with an insulating layer interposed in between the circuit pattern and the metal substrate. Moreover, when higher thermal conductivity is required, the metal base substrate is made of a copper board, which is directly bonded to a ceramic substrate made of, for example, alumina or aluminum nitride. For an application requiring relatively small electric power, a metal base substrate is generally used. In this case, however, in order to improve the thermal conduction, the insulating layer must be thin. Therefore, the spacing between the circuit pattern and the metal plate is susceptible to noise, and the withstand voltage is insufficient.
For avoiding such problems, references have suggested substrates made by integrating lead frames as electrodes and compositions containing resins filled with fillers having good thermal conductivity. For example, JP-A-10-173097 (1998) suggests a substrate comprising such a composition. A method of manufacturing the thermal conductive substrate is shown in FIGS. 7A and 7B. According to the reference, a sheet of a thermal conductive mixture 72 is manufactured from a slurry of a mixture containing an inorganic filler and a thermosetting resin. After a drying step, the sheet of the thermal conductive mixture 72 is laminated on a lead frame 71 as shown in FIG. 7A, and then, the laminate is cured by applying heat and pressure to provide a thermal conductive substrate 74 comprising an electrical insulating layer 73 as shown in FIG. 7B.
In general, a semiconductor module comprising such a substrate designed for improving thermal radiation is contacted properly with an external thermal radiation member in order to transfer heat generated at the semiconductor and respective components to the external thermal radiation member, so that the temperatures of the semiconductor and the respective components will be kept not higher than a certain level. For this purpose, thermal radiation boards having high thermal conductivity often will be provided to the substrate on a surface to be contacted with an external thermal radiation member.
In this case, the thermal radiation board of the circuit substrate and the external thermal radiation member should be fixed firmly to each other, and these components should be contacted sufficiently with each other for reducing thermal resistance therebetween. In general, a semiconductor module is fixed to an external thermal radiation member by a screw or the like. More specifically, screw holes or openings are formed at four corners or sides of a semiconductor module, i.e., at four corners or sides of a circuit substrate, in order to fix the semiconductor module to the external thermal radiation member. For decreasing thermal resistance, the circuit substrate and the external thermal radiation member are fixed after being provided with a thin coating of a silicone compound or the like, since the silicone compound has a relatively good thermal conductivity.
The flatness of a circuit substrate is an essential factor for improving contact and for reducing thermal resistance while fixing the circuit substrate or a semiconductor module using the same to an external thermal radiation member. Considerable warping in the circuit substrate will create a space between the circuit substrate and the external thermal radiation member, and thus, the thermal resistance is increased. Especially when the circuit substrate warps greatly and moves away from the thermal radiation board, the circuit substrate is contacted with the thermal radiation member only at the four corners or sides but not at the central part. As a result, the thermal resistance is raised and the temperature of the module rises so much that abnormal operation and failure of the components result. When the circuit substrate warps and protrudes toward the thermal radiation board, the circuit substrate is contacted well with the external thermal radiation member. However, stress will occur at a time of fixing, which causes fractures and cracks of the insulating layer. Otherwise, peeling occurs between either the wiring pattern or the thermal radiation board and the insulating layer.
Generally, such a circuit substrate of a semiconductor module is fixed to an external thermal radiation member at room temperature. However, since the circuit substrate is constructed by laminating layers of various materials, a warping degree of the board varies depending on the temperature. Therefore, even if the circuit substrate is contacted sufficiently with the external thermal radiation member at the time of fixing, the circuit substrate warps due to the temperature rise during the operation of the semiconductor module so that the contact area is reduced to cause thermal runaway of the semiconductor module.
When warping of the board varies considerably depending on the temperatures, stress on the board is increased at a time of fixing the board to an external thermal radiation member. As a result, fractures or cracks occur in the board, which results in insulation failure or degradation in the reliability.